What is the main function of the part labeled Yin the model?

The part labelled Y is the chloroplast and its main function is to produce food for the plant in the reactions of photosynthesis.

Explanation:

Plants are autotrophic organisms in that they are able to manufacture their own food. Plants are able to manufacture their own food due to the presence of the cell organelle known as the chloroplast.  The plant chloroplast has its own DNA and can reproduce independently, from the rest of the cell. They can as well produce amino acids and lipids required for the chloroplast membrane.

Chloroplasts are green-colored membrane-bound organelles found within the plant cell. Their green color is due to the presence of the pigment, chlorophyll. Chlorophyll in the chloroplasts are responsible for absorbing light energy from sunlight and using this absorbed energy to make simple sugars (carbohydrates) from carbon (iv) oxide and water. This is the main function of the chloroplasts in plant cells.

Photosynthesis occurs in the chloroplasts in two reaction steps:

1. The light reaction - involves the splitting of e=water molecules using the energy of sunlight

2. The dark reactions - carbon (iv) oxide from the air is used to produce sugars.